Recently, there is demand for an electronic integrated circuit which can be driven with a low voltage, with high frequency, and reduced noise. Therefore, there is also demand for a solid electrolytic capacitor having a low ESR (equivalent series resistance), and a low ESL.
In order to produce an anode electrode which is made of niobium or tantalum and used for a solid electrolytic capacitor, a dielectric oxide film is formed on a sintered body which is prepared by pressing aggregates having a size in a range from 40 to 100 μm so that about 70% by volume of pores remain which is relative to 100% by volume of pores before pressing and sintering. In order to produce these aggregates, first, primary particles of niobium or tantalum are heat aggregated by, for example, a method in which potassium fluorides are deoxidized using an oxygen scavenger, such as magnesium.
Recently, in order to increase the capacity of the capacitor, primary particles of niobium or tantalum have been made finer.
When the primary particles becomes finer, pores of a molded product or sintered body made of such primary particles becomes finer. As a result, an electrolyte solution cannot sufficiently permeate over the entirety of the inside of the primary particles, and a problem causes that the ESR cannot be reduced. At the present time, the pore diameter of a practical sintered anode electrode which is made of niobium or tantalum and has about 50,000 CV is about 0.4 μm. In order to increase the capacity and lower the ESR, it is essential to solve the problem of insufficient permeation of an electrolyte solution due to fineness of the pore.
Several methods for improving the permeability of an electrolyte solution into a sintered anode electrode have been suggested.
For example, Unexamined Japanese Patent Application, First Publication No. Hei 09-74051 discloses a method in which a mixture in which polymers having a string shape are added and mixed to aggregates of tantalum and the like is molded and sintered, and thereby micro pores are formed in the sintered body.
Unexamined Japanese Patent Application, First Publication No. Hei 06-252011 discloses a method in which in order to shorten paths of pores, a thin sintered body is prepared and then a layer is formed of plurality of the thin sintered bodies. According to the method, it is possible to improve permeability of manganese nitrate into sintered body.
In addition, a method has also been suggested in which large pores remain by reducing the pressure applied to form a molded product.
However, the method disclosed in Unexamined Japanese Patent Application, First Publication No. Hei 09-74051 provides a sintered body having pores between a plurality of aggregates, and cannot provide a sintered body in which each aggregate comprises pores. Therefore, permeation of an electrolyte into the pores between a plurality of aggregates may be improved; however, permeation of an electrolyte inside of each aggregate is not improved. In addition, since carbon generated from added polymers remains inside of the aggregates, this causes adverse effects on the capacitor properties.
The method disclosed in Unexamined Japanese Patent Application, First Publication No. Hei 06-252011 has the problem that a number of required processes increases.
In the method in which large pores remain by reducing the pressure applied to form a molded product, there is a problem that the adhesive strength between an electrode wire and a molded product which are used for an anode electrode decreases.